A method for manufacturing a semiconductor device, which has a low resistance semiconductor substrate to reduce an ON-state resistance of the device, is disclosed in Japanese Patent No. 2513055 (i.e., U.S. Pat. No. 5,242,862). This device is a vertical type power electronic device. The device includes a semiconductor substrate and a backside electrode. The backside electrode is formed on a backside surface of the substrate. A device layer is formed on a foreside surface of the substrate, which is opposite to eth backside surface.
Before forming the backside electrode on the backside surface of the substrate, the backside surface of the substrate is polished so that the thickness of the substrate becomes small. Therefore, the resistance of the substrate is reduced, so that the ON-state resistance of the device is reduced.
However, for example, the substrate 1 is a silicon substrate with a six-inch diameter, i.e., a six-inch silicon wafer, and having a thickness of 600 μm, and the substrate is polished roughly. The substrate may bend easily, when the thickness of the substrate becomes smaller than 300 μm. Therefore, it becomes difficult to form the backside electrode on this bent substrate.
On the other hand, when the substrate is polished sensitively, the substrate is limited from bending. However, an amorphous silicon layer is formed on the backside surface of the substrate, which is the polished surface. Therefore, since the amorphous silicon layer is disposed between the substrate and the backside electrode, the backside electrode does not connect to the substrate with ohmic contact. Thus, the contact resistance between the backside electrode and the substrate is increased. Further, since the surface roughness of the backside surface becomes small in case of polishing sensitively, a contact area between the substrate and the backside electrode becomes small, so that the contact resistance therebetween becomes larger. Furthermore, the strength of adhesion therebetween is reduced, so that the backside electrode may be peeled off from the substrate 1.